I actually have a solution in mind for this.
Under the current proposal, a method containing super is "statically bound" to a specific object that provides the [[Prototype]] base for the "super" property lookup. This is normally done by defining the method in the context of an object literal or class declaration. If you want to define a method containing outside such a context you still need to provide the necessary binding.
What I have in mind is adding a reflection function for doing that:
let f= function () {return super.f()};
f(); //TypeError -- super unbound
let sup = {foo() {return "super hello"};
let sub = sup <| {};
Object.defineMethod(sub,"foo",f}; //adds f as property named "foo" of sub. binds super for f
f(); // super hello